The start battery is your Auxiliary battery, it's wired up like they used to years ago. Thats one reason I didn't go for the second battery, I just have a lithium Auxiliary battery charged via a dcdc charger and added charge points come from this, my next challenge is to get the roof points changed over to the 2nd batteryOddly I was doing exactly the same test at the weekend with a fridge plugged into the supplied 12v supply in the boot and got very similar results, ( Start 12.4 main 12.4 aux , after 3.5hrs 11.9 main, 12.4 aux). the "Main" battery showed a draw but the "Aux" didn't move, engine off and the aux power button on.
I assumed I got confused, as was doing other things and this test was a side line, but you've got me wondering now too, what's drawing from what?
More investigation required.
G.